Schematic is for Flash EPROM-s of 1 megabit, because they are
easiest to find, and cheapest. You may even take such chip from
damaged PC motherboard for free... I gave 3 types of Flash Eprom-s
as possible example. Important is that it support page-write
operations. Usual page size is 128 bytes. By such Eprom-s is not
required erase of whole chip or block before programming. It is
very welcome for changes and experimenting with Spectrum's Basic
and Bios.
Chip must be inserted in socket so that pin 16 of IC goes to pin
14 of socket. Pins 1-2 and 31-32 then will be in 'air'. They must
be connected with soldering wires. Pin 1 stays unconnected. It's
good to put ceramic capacitor of 100nF between pin 32 and GND.

About ROM-switch: it is best to use programmable ROM-switch
like this one from my site. In that case desired ROM section can
be choosen by simple OUT command. It can be made with mechanical
swithes, or jumpers too.